Sodium hypochlorite (NaOCl) sometimes referred to as "household bleach", or, simply "bleach", is a standard disinfectant and biocidal agent, used in the home, and in industry, such as medical settings.
In solution, the sodium hypochlorite dissolves, forming, inter alia, the hypochlorite, or "OCl" ion. It is this hypochlorite ion which is the disinfecting or biocidal agent. The wide availability and low cost of NaOCl, as well as its ready solubility, have made it one of, if not the, preferred disinfecting/biocidal agent, especially in industry. Its efficacy has been studied, and as a result, regulatory and quasi-regulatory authorities have determined that a minimum concentration of 0.525%, or 5250 parts per million ("ppm"), of NaOCl, is necessary to meet disinfecting standards. Any solution with a strength less than this is not considered to meet required standards.
While it may, at first glance, seem easy to meet the minimal standards provided supra, there are issues with preparing these solutions. First, sodium hypochlorite is also an irritant, and can be toxic. "Chlorine eyes", the phenomenon of eye irritation from a swimming pool with a chlorine concentration that is too high, is one example of the irritation caused by concentrations which are too high. Further, it is simply not economical to prepare solutions where the concentration of the active ingredient is too high. As a result, there is a risk, when working with low concentrations of active material to dilute these to a point where required standards are not met.
Perhaps the most problematic issue in this area, however, is the aging of hypochlorite solutions. It is standard practice to prepare a large quantity of a bleach solution at a point in time and then to use it, on an as needed basis, rather than to prepare it when needed. This solution preparation can take place hours, or days, before the bleach is actually used. Sodium hypochlorite is less stable than water, and decomposes As a result, a solution which may contain a disinfectant/biocidal effective amount of hypochlorite when it is prepared may very well be below this standard at the point in time when it is used. Use of such solution fails to comply with sanitary guidelines, and may not result in effective disinfection of the material it is used upon.
There is a substantial body of literature in the determination of hypochlorite, and free chlorine, in samples. At the start, it must be noted that terminology in this field is not uniform, and it is not always clear as to what chlorine containing ion a particular reference is addressing. For purposes of this disclosure, hypochlorite is the OCl.sup.- radical, and is the only determinant under consideration This is sometimes referred to as free chlorine or available chlorine in the relevant literature. U.S. Pat. Nos. 4,322,215 and 4,391,775 to Huber et al., are related as parent and divisional of each other. These references explain how it is important to distinguish between different species of chlorine in, e.g., drinking water. Hypochlorite it is explained, reacts with hydrogen peroxide, in a reaction which eventually yields free hydrogen, oxygen, and a chloride ion. This reaction, while of potential value in a completely liquid system, is not relevant to the invention described herein, for reasons which will be explained infra. In brief, hydrogen peroxide is unstable, and deteriorates upon standing. Hence, it is ineffective as a material needed to be impregnated into an analytical device, or used in a storage stable solution.
U.S. Pat. No. 5,491,094 to Ramana et al, discusses test strips which can be used in determining free chlorine in solutions. the patent discusses how chloramine, which is frequently present in solutions which also contain free chlorine, can interfere with the determination of the relevant analyte. As a result, Ramana et al. "buffer" their strips at high pH, i.e., 10 or more.
U.S. Pat. No. 5,300,442 to Frant discusses the reaction of chlorine with iodine, leading to iodide ions. In order to prevent iodide, which is determinable, from reverting back to iodine, which is not, zinc dust is added to the solution.
U.S. Pat. No. 5,229,270 to Ono shows a complex, enzyme activating reaction, to determine chlorine in solution. Such a system is not practical for the type of systems contemplated herein, and is not useful on a day to day basis.
U.S. Pat. No. 5,096,721 to Levy teaches that sodium thiosulfate can react with and inactivate free chlorine. The thiosulfate is added to the lining of a paper cup, so that when water is added thereto, any free chlorine is inactivated. This is not an analytical device of the type described herein, however.
U.S. Pat. No. 4,092,115 to Rupe teaches detection of free chlorine ions in a sample via the use of particular indicators. This is not unlike U.S. Pat. No. 5,155,048 to Williams et al., which teaches various aminoalkyl/alkyl ring containing indicators which are useful in determining chlorine in solution. An older patent, i.e., U.S. Pat. No. 3,937,613 to Rosicky, teaches immersion of test strips into a swimming pool, after which reagents disperse therein. If the pool water turns pink, then chlorine is present. Intensity of the pink color is used as a determination of how much free chlorine is present.
What is absent from any of these references is a teaching or suggestion of a "cutoff" determination system, i.e., one which only detects the analyte of interest, hypochlorite, if the analyte is present at a sufficiently high concentration, i.e., one that meets standards for disinfection. As defined herein, this is a concentration of at least 0.525%, or 5250 ppm. Determining that hypochlorite is present per se is not particularly useful in the situations described herein because, as noted supra, at concentrations below the defined minimum, the required disinfecting/biocidal properties of hypochlorite cannot be assured. Further, many of the systems described in the references simply are not economical, or practical, for day to day industrial or home use. Essentially, the invention involves apparatus and reagents which can be used simply, efficiently, and provides an answer to the question of whether or not minimum effective concentrations of hypochlorite are present, in two minutes or less. In the case of the apparatus of the invention, such as test strips, these are dry materials which can be used, e.g., by placing one or a few drops of the test solution thereon, and then reading the test strips in less than two minutes. In the reagent compositions of the invention, the same approach can be taken, i.e., the solution to be tested is combined with the reagent, and color formation is determined, against a neutral background such as white paper, in two minutes or less.
The inventive aspects and preferred features of the invention will be seen in the detailed description of the invention which follows.